The decay of transcripts ina the nucleus caused by prematuare termination condoms (ptcs) has drawn a lot of interest because of its potential relevance to disease. This proofreading response may play an important role in the immune system and other biological systems because it would inhibit the expression of potentially deleterious truncated proteins translated from ptc-bearing mRNAs. The molecular mechanism of these unusual proofreading response is not understood. As a model system to study this proofreading response, the Wilkinson laboratory has used the T-cell receptor (TCR) gene, since this gene grains ptcs during normal thymic ontogeny as a result of a programmed gene rearrangement process. Using the TCR gene system, it has been shown that two signals are necessary to engage the proofreading response; 1) a nonsense codon and 2) a downstream intron. My preliminary data further support this observation. In addition, my preliminary data suggest the ptcs cause exon skipping. These observations provide strong evidence that the proofreading response involves nuclear events (since splicing of introns occurs in the nucleus). In the present application. I plan to test two hypotheses: (1) the rate of splicing of the intron(s) downstream of a nonsense codon dictates whether a ptc will deliver a downregulatory signal or not; and (2) nonsense condoms trigger exon skipping in a manner that permits the accumulation of a spliced transcript lacking the ptc. These two specific aims address the surprising connection between translation like events and RNA splicing. Elucidation of the underlying molecular mechanism responsible for this connection is likely to alter the prevailing view of gene expression.